The Combine are frequently shown as harsh rulers over the citizens of Earth, suppressing dissent with brutality, policing using violence and using invasive surgery to transform humans into either soldiers or slaves. Throughout the games, the player primarily battles with transformed humans as well as synthetic and mechanical enemies that are the product of Combine technology. The atmosphere generated by the dystopian Combine state has been praised by reviewers, although the artificial intelligence of the transhuman Combine characters was thought to be inferior to that of other characters in Half-Life 2. In addition to their role within the Half-Life series, the Combine have been adapted for machinima productions and one Combine character type has been made into plush toys by Valve.

Some elements of the Combine's appearance, such as that of the Advisor, are inspired by the works of Frank Herbert.[4] The name "Combine" itself is a tribute to Ken Kesey's novel One Flew Over the Cuckoo's Nest, which features a collection of authorities which mechanistically manipulate and process individuals.[4]

During Half-Life 2's development, various concepts for Combine non-player characters were created and later cut. Female Combine assassins, similar to black operations assassins featured in the first game, were planned but later abandoned, although they appear in the Half-Life 2: Survivor arcade game.[4] Another non-player character, the cremator, was conceptualized as a Combine laborer who cleaned the streets of bodies after a battle with an acid gun and although removed from the game, its head was featured in Eli Vance's laboratory. Other cuts included a variety of alien Combine soldiers that would have complemented the transhuman soldiers in the game and a number of synthetic combat machines.[4] Many of Half-Life 2's Combine characters went through multiple redesigns; the Combine Overwatch soldier was subjected to at least twelve redesigns before the final appearance was settled on.[5]

Opera singer and actress Ellen McLain provides the voice for the Combine Overwatch announcer and dispatcher in Half-Life 2 and its episodic expansions,[6] while the various Combine soldiers throughout the games are voiced by John Patrick Lowrie.[3]

Little is revealed of the Combine's role outside of Earth, but dialogue in Half-Life 2 states that they control worlds in different dimensions and inhabited by a range of species.[7] The Combine occupation of Earth, however, is shown to be a brutal police state. In City 17, an Eastern European city, Civil Protection units are seen frequently, often conducting random searches of apartment blocks, interrogating human citizens and engaging in random police brutality. The military Overwatch forces of the Combine are shown attacking human resistance bases in an effort to further solidify their control. The citizens themselves are all clad in blue uniforms and live in designated apartment blocks. Citizens are shown to be moved around to different cities or locales at the Combine's will, using passenger trains.[8]Vortigaunts, alien creatures from the original Half-Life, are also shown to have been enslaved, and are observed in various jobs such as janitors. According to Half-Life 2: Raising the Bar, the Combine are draining the Earth's oceans, minerals and resources to be used on other Combine worlds;[4] both the water levels along the coast of City 17 and later coastal levels are also greatly reduced.[9]

At the heart of the Combine's command structure in Half-Life 2 is the Citadel, an enormous structure of Combine construction, reaching approximately 2.6 kilometres (1.6 miles) into the sky from ground level,[10] and delving deep underground. Located within City 17, the Citadel serves as the primary headquarters of the Combine, housing both Combine Advisors and the office of the Earth administrator, Wallace Breen. Breen is frequently seen on large screens around the city from which he spreads propaganda. The Citadel projects an energy field that is able to prevent human reproduction,[11] as well as a field that keeps dangerous alien wildlife out of the city.[12] In addition, the Citadel contains a trans-dimensional teleporter which allows the Combine to travel between their native universe and Earth.[13] The Citadel also contains construction facilities for various synthetic Combine combat machines.

The Combine is composed of various species and machines. The most commonly encountered enemies throughout Half-Life 2 and its expansions are the transhuman Overwatch soldiers and human Civil Protection officers. A variety of combat machines also encountered, ranging from APCs and helicopter gunships to a giant 'Smart wall' which encloses occupied cities and gradually destroys anything in its path. A number of weaponised alien 'synths' are also encountered by Freeman.[14]

The Advisor's grub-like appearance was inspired by the works of science fiction author Frank Herbert.

Advisors are large larvae-like creatures which are virtually featureless, with no visible eyes, ears or limbs, though they do possess an eye-like mechanical device attached to the left side of their heads, and detachable mechanical arms.[15] Their faces are covered by a form of respirator, which is able to lift to reveal a mouth-like orifice from which extends a long flexible proboscis. With this, they can examine objects, or attack and kill enemies. Advisors appear to be feeding upon their victim during their attacks. It is implied that Advisors are the original master race behind the Combine, with the Earth administrator Wallace Breen answering directly to them.[15][16] Although Advisors are usually seen in protective pods guarded by Combine soldiers, they also possess telekinetic powers with which they are able to float through the air and immobilize enemies so that their proboscis can examine their victims without interference.[15] Their appearance was based on the Guild Navigators from the film Dune.[17]

Civil Protection is the Combine's law enforcement agency on Earth, consisting of humans who have agreed to work on behalf of the Combine. Also referred to as metrocops, Civil Protection units wear light armor and gas masks. They are usually armed with electroshock batons and pistols, and are occasionally seen using submachine guns. Metrocops are brutal in their methods, keeping the local populace in line via intimidation and physical force. Interrogations, inspections, raids, random beatings, summary executions and acts of extreme police brutality are all used as a means of policing their respective jurisdictions; their methods are justified by their role as "protectors of the civilians' well-being". On the outskirts of City 17, Civil Protection units are observed patrolling for escapees from the city. Civil Protection officers are in constant contact with Combine Overwatch headquarters, which gives them objectives and situation updates. On the death of an officer, that officer's armor detects that its wearer is no longer alive and automatically informs headquarters of the fatality and advises nearby units as to the place of death.[14]

The Combine Overwatch is the primary military force of the Combine on Earth. It consists of biomechanically enhanced humans, who wear heavily padded long-sleeve body armor and gas masks. The markings on the suit change depending on each individual unit's assignment and rank. Elite Combine soldiers wear bright white armor with one red eye, while the majority of soldiers wear blue armor. Shotgun units wear similar armour to the standard Combine Soldier, but coloured brown with orange eyes. They are armed with a variety of firearms, including shotguns, submachine guns, sniper rifles and pulse rifles. Overwatch soldiers usually operate in small groups, using squad tactics and grenades to flush out and flank the player. They occasionally provide support to Combine synths, and often travel to areas by use of dropships. They use radios to communicate with each other and with Overwatch headquarters. Combine Overwatch soldiers are usually encountered outside City 17, and only appear in the city after "The Uprising" near the end of Half-Life 2.[14]

The Combine use a large array of science fiction technology. They have access to teleportation technology, which takes them from their dimension to Earth. However, their teleporter technology is restrictive in comparison to that developed by Eli Vance, Isaac Kleiner and Judith Mossman in that it cannot be used to teleport to other locations on Earth without significant modification.[18] Throughout the games, various futuristic computer consoles, doors, power sources and weapon emplacements are encountered. In addition, the Combine employ the use of small airborne robots, scanners, to observe the citizens of Earth. The city variant merely moves about the streets, monitors individuals, and takes photographs, while the combat equivalent is capable of dropping mines into an area.[14] Civil Protection makes use of smaller airborne robots called manhacks, which fly using razor-sharp rotating blades with which they attack their targets to cause laceration injuries. These are often deployed in closed-in areas. The Combine also use two types of land mines; hopper mines throw themselves into the air and detonate when an enemy is detected nearby, while the spherical rollermines roll towards vehicles or other enemies, attach themselves and deliver damaging electrical attacks.[14] Combine technology is also used to transform humans into Overwatch soldiers or stalkers, deformed and mutilated humans with no memory of their past selves who act as slaves and maintenance workers in Combine facilities. Transformation into a Stalker is considered among the Combine's worst punishments for dissidents.[15][19]

Militarily, the Combine make use of both synthetic machines—machines powered by organic components—and traditional mechanical machines such as armored personnel carriers and attack helicopters.[14] The most prominent of the synthetic machines are the insect-like gunships; and Striders, 50-foot-tall (15 m) armored creatures that walk upon three legs, which are armed with a high powered warpspace cannon and a head-mounted pulse turret.[14] In Episode Two, a smaller equivalent to the Strider, the Hunter, is introduced. These 2.5 m (8 ft) tall, tripodal assault machines fire explosive flechettes at targets and are small enough to maneuver indoors. Other synths are seen near the end of the Half-Life 2, although their roles are not elaborated on. Sentry turrets are also used by the Combine.[14] The Combine also use headcrabs as a biological weapon against dissidents, firing artillery shells loaded with the creatures into areas and allowing them to infest the vicinity.[14]

According to the backstory presented during Half-Life 2, the Combine appear on Earth after the death of the Nihilanth, the boss character at the end of Half-Life. The death of the Nihilanth, a powerful creature controlling the dimensional rip between Xen and the Black Mesa Research Facility on Earth, causes the rip to worsen, resulting in "portal storms" which spread the hostile wildlife of Xen across Earth. The Combine manipulate this tear in the spacetime continuum, widening it to allow access to Earth from their dimension. When it is sufficiently wide, the Combine invade in force, and in the subsequent Seven Hour War quickly defeat the forces of Earth and destroy the United Nations Secretariat Building in New York City. Earth's surrender is negotiated by Wallace Breen, who is then made the Combine puppet administrator of Earth, and uses City 17 as his base of operations.[20]

Two Overwatch soldiers in Episode Two, with a Hunter in the background.

The Combine's first appearance is in Half-Life 2. Through the early stages of the game, Combine Civil Protection units pursue Gordon Freeman through City 17 after he accidentally reveals his presence to Wallace Breen. Due to Freeman's actions in Half-Life, Breen sees Freeman as a major threat.[21] As Freeman escapes the city, Civil Protection units raid the resistance base of Black Mesa East and capture resistance leader Eli Vance, who is transferred to holding facilities at Nova Prospekt.[20] Freeman and Vance's daughter Alyx break into the facility to rescue him, but Eli Vance is teleported to the Combine Citadel by double agent Judith Mossman. However, the strike against Nova Prospekt is seen as the start of a revolution by the citizens of Earth, who take up arms against the Combine.[22] In response, Combine Overwatch are deployed to City 17, and heavy street fighting takes place. Freeman manages to infiltrate the Citadel but is captured and taken to Breen. Mossman allows Freeman to break free of his captivity, and Breen flees to the Citadel's trans-dimensional teleporter, intent on escape. However, Freeman destroys the teleporter's reactor before Breen can escape, causing a large explosion that destroys the top of the Citadel.

In Episode One, the destruction of the top of the Citadel has caused its primary reactor to begin to melt down, forcing Gordon Freeman and Alyx Vance to journey back into the Citadel to temporarily stabilize it while the city's inhabitants are evacuated.[23] The Combine, however, attempt to accelerate the meltdown to send a message to their native universe for reinforcements. After Alyx acquires an encrypted copy of the data to be sent, Overwatch forces desperately attempt to stop the pair from escaping the city, spurred on by Combine Advisors. As the pair escape on a train at the end of the game, the Citadel sends the message and detonates, destroying City 17.[24]

Episode Two opens with Alyx and Freeman discovering that the Citadel's destruction has resulted in the formation of a super portal to the Combine dimension, which is slowly progressing to a stage where it can send overwhelming reinforcements.[25] They also discover that the data packet they retrieved from the Citadel contains codes that can be used to close the portal, and so traverse the countryside to deliver the packet to the resistance headquarters at White Forest. As they progress, it becomes evident that the Combine Advisors escaped the Citadel's destruction, and that remaining Combine forces are regrouping, albeit under occasional attacks by Vortigaunts.[26] Aware of the resistance's plans to close the super portal, the Combine attack White Forest in force, but are repelled.[27] The super portal is destroyed after the resistance launch a satellite containing the appropriate data. However, in the game's closing scenes, two Advisors attack Freeman, Alyx and Eli Vance, and kill Eli before being driven off by Dog.

The use of sandbox applications like Garry's Mod have allowed for Combine non-player characters to be used in a variety of webcomics and machinima productions. In one webcomic, Concerned, the Combine are portrayed as a highly bureaucratic and often inept organization. One issue shows a Civil Protection briefing for attempting to capture the comic's protagonist Gordon Frohman, in which officers are instructed to cluster around explosive barrels, seek cover on unstable structures and rappel down from bridges in front of fast moving vehicles.[28] In another example, the machinima series Combine Nation follows Civil Protection officers in a similar style to police procedural documentaries. The officers perform their duties with various twists, such as the team's medical officer having an obsession with adhesive bandages and the legal consultant, an Overwatch soldier, favoring dramatic entries, such as throwing flashbangs, which often backfire on him.[29] Another web series, called The Combine, parodies the TV show The Office. This series attempts to portray the combine as an intergalactic corporation that suffers from everyday office issues and problems. One episode shows the soldiers attempting to bypass a webfilter so they can watch videos online instead of working.[30] Other media portray the Combine with more serious overtones, such as the live-action video The Combine Interview, which parodies an interview with Tom Cruise discussing Scientology. The video, described by ActionTrip as "eerie, to say the least"[31] and by both Joystiq[32] and Kotaku[33] as "creepy", instead presents an interview with a Civil Protection officer discussing the Combine's rule of Earth, adapting Cruise's words to fit the Combine theme. PC Gamer UK noted that "the suggestion, of course, is that Scientology's purpose or self-image in some way resembles that of the homogenising intergalactic murderous alien collective".[34]

The Combine have inspired the creation of several items of merchandise for the Half-Life series. A plush toy was created by Valve, based on the synthetic tripod Hunters introduced in Episode Two.[35] Sold and distributed via Valve's online store, the toy was released in February 2008.[36] In addition, Valve has produced t-shirts depicting the Combine's idea of humanity's evolution, from ape to Combine Overwatch soldier,[37] and a lithograph displaying twelve pieces of concept art for the Combine soldier.[5]

The Combine have received a positive reaction from critics. 1UP.com praised the "epic feel" built up by the Combine and their harsh rule of City 17 in Half-Life 2, stating that this created "a world governed by newspeak, decorated with urban decay, and lacking any hope".[38]GameSpot echoed this praise, saying that the "vision of a dystopian police state is chillingly effective". PC Zone described the appearance of the Combine's soldiers as "stormtrooper-like", but although stating that overall the artificial intelligence for the game was "extremely competent", the Combine non-player characters "could have used better survival instincts", citing their reluctance to take cover and tendency to charge at the player and into a shotgun blast.[39] A number of reviews of Episode One were disappointed by the lack of new characters for the Combine,[40] although GameSpot praised the improved AI for Combine soldiers and the addition of new abilities, such as rappelling down buildings. [41] The addition of the Hunter in Episode Two was particularly well received by critics; Computer and Video Games stated that they were challenging to fight and were "a very welcome addition" to the series,[42] while IGN stated that the Hunters were "impressively designed... [they are] sleek and powerful all at once and reek of malicious alien intelligence."[43]

The Combine were ranked at #22 in IGN's 'Top 100 Videogame Villains' list.[44]

^Valve Corporation (2006). Half-Life 2: Episode One. PC. Level/area: Urban Flight. Isaac Kleiner: I feel obliged to point out that a more fortunate side effect of the reactor's destruction is the complete removal of the Combine's reproductive suppression field. Previously, certain protein chains important to the process of embryonic development were selectively prevented from forming; this is no longer the case. For those so inclined, now would be an excellent time for procreation, which is to say, in layman's terms, you should give serious consideration to doing your part for the revival of the species.

^Valve Corporation (2004). Half-Life 2. PC. Level/area: Dark Energy. Dr. Wallace Breen: (to an Advisor) It's me you should be worried about! I can still deliver Earth, but not without your help! [..] The portal destination is untenable, surely you can set the relay else where. There's no way I can survive in that environment! A host body?! You must be joking, I can't possibly—oh, all right, dammit, if that's what it takes. Just hurry, he's right behind me!

^Valve Corporation (2004). Half-Life 2. PC. Level/area: Black Mesa East. Judith Mossman: We're closing in on a reliable local teleport technology, something the Combine still hasn't mastered. Eli thinks their models are string-based, similar to our Calabi-Yau model, but they fail to factor in the dark energy equations. They can tunnel through from their universe, but once they're here, they're dependent on local transportation.

^Valve Corporation (2006). Half-Life 2: Episode One. Level/area: Direct Intervention. Alyx Vance: Goddamn the Combine! This is what happens to you if you resist... or if you're just in the wrong place at the wrong time. [..] God, I hope you don't remember who you are...

^Valve Corporation (2004). Half-Life 2. PC. Level/area: Water Hazard. Wallace Breen: We now have direct confirmation of a disruptor in our midst, one who has acquired an almost messianic reputation in the minds of certain citizens. His figure is synonymous with the darkest urges of instinct, ignorance and decay; some of the worst excesses of the Black Mesa incident have been laid directly at his feet. And yet unsophisticated minds continue to embue him with romantic power, giving him such dangerous poetic labels as the "One Free Man", the "Opener of the Way".

^Valve Corporation (2007). Half-Life 2: Episode Two. PC. Level/area: To The White Forest. Isaac Kleiner: What you're seeing is the infancy of a superportal. If it retains full strength— / Eli Vance: It will be the Seven Hour War, all over again! Except this time we won't last seven minutes!

^Valve Corporation (2007). Half-Life 2: Episode Two. PC. Level/area: Our Mutual Fiend. Resistance fighter: We've counted a dozen Striders just north of us. Our job is to keep them from reaching the base to the south. If they get close enough for one good shot at the silo, the whole launch is a bust. And in case Striders aren't bad enough, recon indicates that they're being escorted by packs of Hunters.